Harmonic distortion limits the usefulness of signal processing systems employing an analog-to-digital converter ("ADC"), such as spectrum analyzers and cable testers, because undesirable spurious signals are introduced at harmonic frequencies of an input signal.
A prior method of reducing ADC harmonic distortion employs adding high-level pseudo-random noise ("PRN") to the ADC input signal and subsequently subtracting the PRN from the ADC output signal. However this approach is disadvantageous because the high-level PRN reduces the maximum input signal amplitude that can be applied to the ADC, and the PRN is difficult to completely remove from the ADC output signal. U.S. Pat. Nos. 4,448,803, 4,550,309, 4,761,634, 4,963,881, 5,134,399, 5,168,459, 5,187,481, and 5,189,418 describe various techniques for removing the PRN from the ADC output signal or reducing distortion by related methods generally referred to as dithering.
Harmonic distortion can be introduced by many components, not just the ADC, of signal processing systems. For example, spectrum analyzers, cable testers, and other frequency-domain test sets typically employ amplifiers, filters, mixers, and local oscillators to acquire an input signal and further employ an ADC to digitize the acquired input signal for processing and display. Harmonic distortion in any of the acquisition and digitization stages can cause spurious signals to be displayed. Because it is very difficult to reduce harmonic distortion to negligible levels, prior workers have developed techniques for coping with the spurious signals. For example, U.S. Pat. No. 4,568,878 for SPECTRUM ANALYZERS, assigned to the assignee of this application, describes a local oscillator frequency shifting method that facilitates the identification and removal of spurious signals. However, such a method operates on predetermined spurious signals and does nothing to fundamentally reduce the overall level of spurious signals generated and displayed.
What is needed, therefore, is a system and method for reducing the amplitude of spurious signals in a system employing a frequency-conversion circuit and an ADC.